1. Field of the Invention
This invention relates generally to a motor output shaft positioning device for accurately positioning the motor output shaft in the axial direction of the shaft, and more particularly to a motor output shaft positioning device for minimizing the axial movement of the rotor and preventing mechanical noise caused by the friction and play of the motor output shaft by accurately positioning the motor output shaft in the axial direction.
2. Prior Art
In general, the axial movement of a motor output shaft is adjusted by using a nut, a screw having a tip which is machined into a special shape, and a steel ball, rotating the motor in a state where the screw comes in contact with an end of the output shaft of the rotor, fixing the screw by tightening the nut at a location where no-load current and the axial movement of the shaft are minimized. Such an adjustment method of the axial movement of the motor output shaft not only involves a considerable degree of skill in fixing the screw and the nut, but also requires much time for the adjustment process.
To solve these problems, a motor output shaft positioning method as disclosed in Published and Examined Patent Application No. 56701/1986, is publicly known.
FIG. 3 shows a cross-sectional view of the essential part of a bearing of assistance in explaining the conventional motor output shaft positioning method.
In FIG. 3, a worm wheel is housed in the lower part (not shown) of a gear case 31, and a motor output shaft 32 fitted to a worm meshing with the worm wheel is supported by a bearing projection 33 protruding radially inside the gear case 31.
At the tip of the motor output shaft 32 formed is a conical projection 34 protruding toward a hollow part 35 defined by the gear case 31 and the bearing projection 33. In the hollow part 35 formed is an injection passage 36 connecting the inside and outside of the hollow part 35.
In this state, a nozzle 37 of an injection molder for injecting a synthetic resin 38 is connected to the injection passage 36 to inject the synthetic resin 38 into the hollow part 35. The synthetic resin 38, which is polyacetal resin, for example, is injected at a lower rate than the normal injection rate. After the resin 38 is injected into the hollow part 35, the surface of the resin 38 is half solidified to form a skin layer 39. In this way, the synthetic resin 38 is prevented from leaking through a gap 40 formed between the motor output shaft 32 and the bearing projection 33.
After that, the synthetic resin 38 is injected under increased injection pressure, allowed to cool and cure to form an appropriate gap between the conical projection 34 of the motor output shaft 32 and the synthetic resin 38.
Because injection pressure has to be increased only after the skin layer 39 has been formed as the synthetic resin 38 that was first injected was allowed to cool and cure, however, much time and labor are needed and injection pressure or injection temperature has to be precisely controlled.
Furthermore, if a skin layer 39 is not formed adequately during the first injection, the synthetic resin 38 may leak from the gap between the motor output shaft 32 and the bearing projection 33.
In addition, extremely troublesome adjustment is needed to maintain accuracies with respect to the relative positions of the synthetic resin 38 injected into the hollow part 35 and the conical projection 34 of the motor output shaft 32, and with respect to the rotatable gap between them.